The present invention relates to an image reading apparatus capable of precisely reading an image region using a simple mechanism.
There is such a conventional image reading apparatus which scans an image medium like an X-ray film with a light beam before converting image data into a digital signal.
Taking a medical X-ray film reading apparatus for example, this apparatus scans an image in the main scanning direction with a laser beam while conveying the X-ray film in the subscanning direction along a conveying guide, and then photoelectrically converts light transmitted through or reflected from the X-ray film before eventually generating an image signal. A photo-detector is provided in the laser-beam scanning width and outside of film conveying path. After detection of scanned light with the photo-detector, based on the predetermined timing, an image-reading section is established.
FIG. 1 illustrates an example of an image-reading-signal generating circuit of a conventional image reading apparatus. The reference numeral 100 designates an address counter which counts input picture-element clock units and then clears the counted value by applying a horizontal synchronizing signal. The reference numeral 200 designates ROM storing data. The reference numeral 300 designates a latch circuit which latches data read out of the ROM 200 and then outputs it as an image signal.
If such data like 111, 1000, and 11, were preliminarily written in the ROM 200 in order that these data can be read out synchronous with picture-element clock, as shown in FIG. 2, after passing a certain time on receipt of horizontal synchronizing signal, image reading signal (shown as negative logic in FIG. 2) can constantly be generated. If image-reading signal generating circuit had the composition like the one cited above, a reading region of a film can be adjusted by applying the timing of an image reading signal which is generated based on the horizontal synchronizing signal. In other words, the reading region can be adjusted by rewriting data in the ROM 200. Once the data of the ROM 200 is determined, based on the stationary timing, the image reading signal is output from the horizontal synchronizing signal. As a result, if the position of film-conveying guide were stationary, image can be read within an accurate range.
Nevertheless, there are a variety of faults in any conventional image reading apparatus. For example, film conveying guide may wobble itself, and yet, depending on the degree of installation precision, image reading position may deviate. If this occurs, an image in some portions cannot properly be read to eventually result in the partial dropout of image data. Conversely, the image reading system may erroneously read the exterior of the film. Even if the image portion were specified by coordinate, since the film position deviates, the specified region cannot precisely be shown. Although such a deviation can be eliminated by applying adjustment during production process, it in turn raises cost. And yet, even if the adjustment were properly done, mechanical structure may wobble itself while operating the image reading apparatus for a long period of time, and then the image reading position will eventually deviate.
The conventional image reading apparatus cited above detects the film size by means of a plurality of microswitches disposed in the main scanning direction at the predetermined intervals. Nevertheless, this system still has those problems mentioned below.
First, the film-size detecting method with a number of microswitches needs to use a large number of component parts. This in turn obliges assembly operators to perform complicated work to install these parts and perform position adjustment.
Next, the size of the film used for medical X-ray photography has been standardized into the following five kinds. The Japanese Laid-Open Patent Publication No. 60-46660 of 1985 suggests that four units of microswitches are just enough to detect the size of the medical X-ray film.
______________________________________ Width in the main Width in the sub- scanning line (inch) scanning line (inch) ______________________________________ 8 10 10 12 11 14 14 14 14 17 ______________________________________
Nevertheless, when reading the size of X-ray films having specific sizes other than the above, more than four of microswitches must be provided, thus resulting in the extremely complex mechanism. Furthermore, when introducing a number of microswitches, reliability and durability of mechanical contacts and actuators must carefully be taken into account. Although either the photointerruption type or the reflection type photo-sensing switches may be introduced in place of microswitches, those problems cited above still remain unsolved. Needless to say that, when introducing reflection-type photosensors, reflection sheet must be provided on the side opposite from those photosensors.